1. Field of the Invention
This invention relates to wireless communication systems, and more particularly to a method and apparatus for efficiently synchronizing MAC and physical communication protocol layers of a wireless communication system.
2. Description of Related Art
As described in the commonly assigned related U.S. Pat. No. : 6,016,311, a wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary communication systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The key objective of these wireless communication systems is to provide communication channels on demand between the plurality of subscriber units and their respective base stations in order to connect a subscriber unit user with the fixed network infrastructure (usually a wire-line system). In the wireless systems having multiple access schemes a time “frame” is used as the basic information transmission unit. Each frame is sub-divided into a plurality of time slots. Some time slots are used for control purposes and some for information transfer. Subscriber units typically communicate with a selected base station using a “duplexing” scheme thus allowing for the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as “downlink” transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as “uplink” transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art.
Recently, wideband or “broadband” wireless communications networks have been proposed for delivery of enhanced broadband services such as voice, data and video. The broadband wireless communication system facilitates two-way communication between a plurality of base stations and a plurality of fixed subscriber stations or Customer Premises Equipment (CPE). One exemplary broadband wireless communication system is described in the incorporated U.S. Pat. No. : 6,016,311, and is shown in the block diagram of FIG. 1. As shown in FIG. 1, the exemplary broadband wireless communication system 100 includes a plurality of cells 102. Each cell 102 contains an associated cell site 104 that primarily includes a base station 106 and an active antenna array 108. Each cell 102 provides wireless connectivity between the cell's base station 106 and a plurality of customer premises equipment (CPE) 110 positioned at fixed customer sites 112 throughout the coverage area of the cell 102. The users of the system 100 may include both residential and business customers. Consequently, the users of the system have different and varying usage and bandwidth requirement needs. Each cell may service several hundred or more residential and business CPEs.
The broadband wireless communication system 100 of FIG. 1 provides true “bandwidth-on-demand” to the plurality of CPEs 110. CPEs 110 request bandwidth allocations from their respective base stations 106 based upon the type and quality of services requested by the customers served by the CPEs. Different broadband services have different bandwidth and latency requirements. The type and quality of services available to the customers are variable and selectable. The amount of bandwidth dedicated to a given service is determined by the information rate and the quality of service required by that service (and also taking into account bandwidth availability and other system parameters). For example, T1-type continuous data services typically require a great deal of bandwidth having well controlled delivery latency. Until terminated, these services require constant bandwidth allocation for each frame. In contrast, certain types of data services such as Internet protocol data services (TCP/IP) are bursty, often idle (which at any one instant may require zero bandwidth), and are relatively insensitive to delay variations when active. The base station media access control (“MAC”) allocates available bandwidth on a physical channel on the uplink and the downlink. Within the uplink and downlink sub-frames, the base station MAC allocates the available bandwidth between the various services depending upon the priorities and rules imposed by their quality of service (“QoS”). The MAC transports data between a MAC “layer” (information higher layers such as TCP/IP) and a “physical layer” (information on the physical channel).
Due to the wide variety of CPE service requirements, and due to the large number of CPEs serviced by any one base station, the bandwidth allocation process in a broadband wireless communication system such as that shown in FIG. 1 can become burdensome and complex. This is especially true with regard to rapidly transporting data while maintaining synchronization between the MAC and physical communication protocol layers. Base stations transport many different data types (e.g., T1 and TCP/IP) between the MAC and physical layers through the use of data protocols. One objective of a communication protocol is to efficiently transport data between the MAC and physical layers. A communication protocol must balance the need for transmitting data at maximum bandwidth at any given time against the need for maintaining synchronization between the MAC and physical layers when the data is lost during transportation.
Prior art communication protocols have been proposed for transporting data in a wireless communication system. One prior art communication protocol teaches a system for transporting MAC messages to the physical layer using variable length data packets comprising headers and payloads. A payload contains data for a MAC message data type (e.g., T1 and TCP/IP). In the prior art, a header starts at a physical layer boundary and provides the wireless communication system with information such as the length of the payload and the location of the next data packet. Typically, the communication protocol provides adequate bandwidth usage via the variable length data packets. However, this type of protocol provides poor synchronization between the MAC and physical layers because when the system loses a header the protocol overlooks all of the subsequent data until it finds the next header at the beginning of the physical layer boundary. The system then begins using data from that physical layer boundary. Thus, the variable length data packet protocol loses a relatively large amount of received data (i.e., the data received between the lost header and the next physical boundary). It is therefore an inefficient communication protocol for use in a wireless communication system.
Another prior art protocol teaches a system for transporting MAC messages using fixed length data packets. In accordance with these systems, a message always begins at a fixed position relative to the other messages. When the system loses a part of a message, the protocol only loses that one message because it can find the next message at the next fixed position. Thus, the fixed length data packet protocol provides adequate MAC to physical layer synchronization. However, the fixed length data packet protocol provides poor bandwidth usage because a fixed length data packet must be sufficiently large to accommodate the largest message from any given data type. As most messages are much smaller than the largest message, the fixed length packet protocol typically wastes a large amount of bandwidth on a regular basis.
Prior art protocols also inefficiently handle the transportation and synchronization of ATM cell data. In typical wireless communication systems, ATM header information is protected from errors using a Header Error Check (HEC) byte. Alternatively, entire ATM cells are protected against errors using either a Forward Error Correction (FEC) or CRC byte. The FEC makes the HEC redundant and thus the HEC is often removed prior to transmission. Disadvantageously, when an error is detected in these systems the entire ATM cell is discarded. This is necessary to prevent the possible misinsertion of the corrupted ATM cell into higher layers of the MAC protocol. However, the discard of entire ATM cells wastes valuable bandwidth and requires the re-transmission of ATM cell data.
Therefore, a need exists for a data transportation and synchronization method and apparatus for efficiently transporting data between the MAC and physical layers in a wireless communication system. The data transportation and synchronization method and apparatus should accommodate an arbitrarily large number of CPEs generating frequent and varying bandwidth allocation requests on the uplink of a wireless communication system. Such a data transportation and synchronization method and apparatus should be efficient in terms of the amount of bandwidth consumed by the messages exchanged between the plurality of base stations and the plurality of CPEs in both the uplink and downlink. In addition, the data transportation and synchronization method and apparatus should rapidly synchronize to the next data message when a part of a message is lost as to prevent a large loss in data. Finally, the data transportation and synchronization method should provide a mechanism for synchronization to ATM cell boundaries. The data transportation and synchronization method should also prevent the misinsertion of ATM cells into the higher MAC communication layers. The present invention provides such a data transportation and synchronization method and apparatus.